Electrodeionization (EDI, Filled Cell Electrodialysis, including Electrodiaresis) is a process for transferring electrolytes (including weakly dissociated electrolytes) from a first fluid to a second fluid under influence of a substantially direct electric potential applied across compartments through which the first and second fluids pass. Generally, the first fluid passes through “dilute” or “diluting” compartments, where ions are removed from the fluid and migrate into adjacent “concentrate” or “concentrating” compartments containing the second fluid. At least some of the compartments, typically including at least some of the diluting compartments, are substantially filled or packed at least in part with ion exchange or other material, such as ion exchange resin beads, fibers, rods, felt or the like.
EDI is particularly attractive to deionize a first fluid such as water which has a low total dissolved solids content, and which exhibits low electrical conductivity that would yield a very high electrical resistance in a conventional electrodialysis arrangement. The use of conductive ion exchange packing in an EDI apparatus provides pathways for conduction of ions through fluid-filled cells and between ion exchange membranes defining walls of the various compartments. Packing also increases surface area available for removal of ions or ionizable matter carried in the first fluid, and constitutes a percolation medium that lengthens residence time of fluid in diluting compartments.
The concept of electrodialysis apparatus containing mixed bed ion exchange material in deionization compartments was apparently first disclosed by Kunin, et.al. (“Ion Exchange Resins”, Wiley, N.Y. 1950, p 109) but no data were given. Chessmore (Master's Thesis, Chem. Eng. Dept., M.I.T., Cambridge, Mass. (1952)), and Walters, et.al. (Ind. Eng. Chem., 47, 61–67 (1955) and “Ion Exchange Technology”, eds. Nachod and Schubert (Academic Press, New York, 1956) were apparently the first to disclose operating data. Other early disclosures were made by Glueckauf, et.al. (e.g., Second United Nations Conference on Peaceful Uses of Atomic Energy, Paper 308 (1958) and Brit. Chem. Eng., 4, 646–651(1959)). Kedem, et al., disclosed filled cell electrodialysis in which dilute compartments were filled with various knit ion exchange fibers (Desalination, 16, 105–118 (1975)), such cells being in the form of a tank having sealed concentrate compartments, open dilute compartments being filled with granular anion exchange resin “which can be poured in and pumped out” (Desalination 24, 313–319 (1978)). In the latter publication, it was suggested that the open dilute compartments may also contain knit cation exchange fibers against cation selective membranes. Flow of fluid through dilute compartments was by gravity, which limited the rate and compartment size to uneconomic values. Such apparatus had the advantage that it could be easily filled with particulate anion exchanger and such exchanger could be easily removed for cleaning or replacement. Concentrate compartments depended solely on electrical transfer of water through surrounding membranes. As a result concentrate was in fact very concentrated and was subject to scaling and precipitation of poorly soluble electrolytes. The same author(s) reported on similar electrodialysis stacks in which dilute chambers were filled solely with a net of multifilament anion exchange material (Desalination 46, 291–299 (1983)). Numerous papers since that time have addressed problems encountered in operation or improvements in construction or filling of EDI and related stacks.
There have been many patent publications concerning packed cell electrodialysis and related processes and apparatus including the following U.S. patents: U.S. Pat. Nos. 2,689,826; 2,815,320; 3,149,061; 3,291,713; 3,330,750; 3,515,664; 3,562,139; 3,686,089; 3,705,846; 3,993,517; 4,284,492; 4,632,745; 4,747,929; and 4,804,451, each of which is incorporated herein by reference. Nevertheless, although electrodialysis with packed cells (e.g., electrodeionization including electrodiaresis) has been known and studied for about 50 years, it has begun to receive widespread commercial use only in the last few years. Reasons for this slow development of commercially competitive EDI devices appear to involve many factors, such as:
a) The need to fill individual compartments in an EDI stack with ion exchange resin material, e.g. particulates such as resin beads, poses the need to keep such resin in place while assembling the stack, and there are practical difficulties in doing this, especially for relatively large stacks. This has typically required very careful hand assembly, or use of sealed layers or modular envelopes of ion exchange particulates, or other relatively costly steps during assembly process of the stack. These constructions, in turn, dictate complete disassembly or rebuilding when it is necessary to change ion exchange packing. Until recently practical external filling and removal of resin particulates has not been done because of lack of any EDI stack designed to be filled with resin particulates after the stack has been assembled; and lack of a process to fill and empty such an assembled stack (e.g., an effective way to move resin particulates into or out of a stack.)
b) Particulate ion-exchange packing is a very good filter medium. Resistance to flow of fluid through packing is increased by material filtered out during operation. In the case of conventional (chemically regenerated) ion exchange deionization, ion exchangers are periodically backwashed at flow rates which expand the volume of particulates, i.e. separating particulates slightly from each other allowing filtered material to escape. However, EDI apparatus typically employs relatively narrow flow passages for diluting and concentrating compartments, constraining flow and particle movement. Until recently such bed expansion capability has not been a feature of EDI apparatus. Instead EDI has been simply preceded by fine filtration to delay or prevent congestion of resin packing. The latter is nevertheless seldom completely effective.
c) Anion exchange particulates tend to sorb negatively charged colloids and medium molecular weight anions which occur naturally in water. Such sorbed materials (generally termed foulants) interfere with satisfactory operation of EDI apparatus, e.g. by increasing electrical resistance and decreasing rate of transport of ions into the interior of particulates. In an EDI process, electric current tends to drive such foulants into anion exchange particulates and thereby accelerate fouling. Until recently EDI stacks have in practice been preceded by scavenging type anion exchange resin and/or activated carbon columns to attempt to remove foulants before they can enter stacks. Such pretreatment is costly and seldom completely effective, especially in view of often unpredictable breakthroughs of foulants upon exhaustion of the resin and/or carbon pretreatment column.
d) Precipitates of sparingly soluble inorganic compounds (e.g., calcium carbonate, magnesium hydroxide, calcium sulfate) tend to form within particulate packing, in anion exchange membranes, or in concentrate compartments of EDI apparatus, when precursors of such compounds are present in fluid processed, because water splitting that continuously regenerates the resin also causes pH conditions that precipitate scale. Such problem does not exist in conventional ion exchange deionization in which anion- and cation-exchange particulates are separately regenerated with alkali and acid respectively. In conventional electrodialysis, build-up of such precipitates may be simply addressed largely by frequent, regular reversal of direct electric current, e.g., a few times per hour.
e) At water dissociating junctions between commercially available anion exchange bodies (e. g. membranes and particulates) and cation exchange bodies, quaternary ammonium moieties (the usual bound positively charged group in commercially available anion exchange bodies) are rapidly converted to tertiary amines and/or non-ionized groups, resulting in increased electrical resistance at such junctions and degradation of stack performance. Such conversion may be due to some combination of high alkalinity, high temperature, and high electric field in the junctions. There is not an equivalent phenomenon in conventional ion exchange deionization under normal process conditions. Thus, in the case of EDI until now it has been necessary after some months to a year or so to disassemble a packed stack and replace at least anion selective membranes and preferably also anion exchange particulates. Some EDI stacks are sealed (i.e. membranes and filled inter-membrane spacers are glued together) in which case it may be necessary to replace an entire stack, possibly with exception of screen-filled concentrate spacers.
f) Electrical resistance of packing depends also on area of contact of beads, hence on deformability of beads, on forces causing such deformation, on distribution of bead sizes and any time dependent relaxation of force, e.g., from cracking of beads. The overall effect is usually a time dependent increase in electrical resistance that may eventually require repair or replacement of a stack. A similar problem does not exist in conventional ion exchange deionization as there is no electric field.
g) Owing to the short distance between membranes in packed electrodialysis apparatus (e.g. about 0.3 centimeters) substantial channeling of processed fluids can occur resulting in less than expected performance.
Methods are taught in U.S. Pat. No. 5,066,375, No. 5,120,416, and No. 5,203,976 to fill stacks after they have been assembled, using a slurry of liquid (e.g., water) and ion exchange particles. These methods have to some extent mitigated one or more of the above deficiencies. Those patents are hereby incorporated herein by reference.
Nonetheless, there remains a need for methods and procedures for depositing an effective filling of particulates in cells of an assembled electrodeionization or related apparatus.
There is also a need for an electrodeionization apparatus that can be effectively filled with, or replenished with, ion exchange packing.